1. Field of the Invention
The present invention relates to an electronic component, a ladder filter and a communication apparatus and, more particularly, to an electronic component, a ladder filter and a communication apparatus each including a plurality of piezoelectric resonators utilizing mechanical resonance of a piezoelectric body.
2. Description of the Related Art
FIG. 26 is a perspective view showing an example of a conventional piezoelectric resonator. A piezoelectric resonator 1 includes a piezoelectric substrate 2, for example, having a rectangular plate. The piezoelectric substrate 2 is polarized in a direction of thickness thereof. Electrodes 3 are provided on both major surfaces of the piezoelectric substrate 2. By inputting a signal between the electrodes 3, an electric field is applied in the direction of thickness of the piezoelectric substrate 2, causing the piezoelectric substrate 2 to vibrate along the length direction. Such a piezoelectric resonator 1 is an unstiffened type, in which the vibration direction is different from the electric-field direction and the polarization direction.
When a ladder filter is to be constructed by using such a piezoelectric resonator 1, for example, the construction shown in FIGS. 27 to 29 is used.
FIG. 27 is a plan diagrammatic view showing an example of the ladder filter. FIG. 28 is a front diagrammatic view thereof. A ladder filter 4 shown in FIGS. 27 and 28 includes an insulator substrate 5. Four pattern electrodes 6a, 6b, 6c and 6d are provided on the insulator substrate 5. One of the electrodes 3 of each of the two piezoelectric resonators 1a and 1b having the same construction as the above-described piezoelectric resonator 1 is connected to the pattern electrode 6b via a conductive bonding agent 7 made of a conductive resin or the like. Also, one of the electrodes 3 of each of the other two piezoelectric resonators 1c and 1d having the same construction as the above-described piezoelectric resonator 1 is connected to the pattern electrode 6c via the conductive bonding agent 7. Further, the other electrode 3 of the piezoelectric resonator 1a is connected to the pattern electrode 6a, the other electrode 3 of the piezoelectric resonator 1b is connected to the pattern electrode 6d, the other electrode 3 of the piezoelectric resonator 1c is connected to the pattern electrode 6b, and the other electrode 3 of the piezoelectric resonator 1d is connected to the pattern electrode 6d, each being connected via a conductive wire 8. A metal cap (not shown) is mounted on the insulator substrate 5 in such a manner as to cover the four pattern electrodes 6a to 6d. This ladder filter 4 has a ladder-type circuit shown in FIG. 29.
FIG. 30 is a plan diagrammatic view showing another example of a ladder filter, which provides a background for the present invention. FIG. 31 is a front diagrammatic view thereof. In a ladder filter 4 shown in FIGS. 30 and 31, each of the piezoelectric resonators 1a to 1d has a multilayer structure.
The features of the piezoelectric resonators 1a to 1d have been described by the applicant of the present invention in, for example, Japanese Patent Application No. 8-110475, which is still not published or laid open. A plurality of piezoelectric layers which constitute a base having a longitudinal direction and a plurality of electrodes are multilayered alternately, and the plurality of piezoelectric layers are polarized in the longitudinal direction of the base, generating a fundamental vibration of longitudinal vibration. Such a piezoelectric resonator of a multilayer structure is a stiffened type, in which the polarization direction, the electric-field direction, and the vibration direction of the piezoelectric layers are the same. In comparison with an unstiffened type piezoelectric resonator, there are advantages including small spurious emissions, and the difference .increment.F between the resonance frequency and the anti-resonance frequency is large. Two external electrodes 3 and 3 are provided on one side of the piezoelectric resonators 1a to 1d. The external electrodes 3 of the piezoelectric resonator 1a are connected to the pattern electrodes 6a and 6b and the external electrodes 3 and 3 of the piezoelectric resonator 1b are connected to the pattern electrodes 6b and 6d, the external electrodes 3 and 3 of the piezoelectric resonator 1c are connected to the pattern electrodes 6b and 6c, and the external electrodes 3 and 3 of the piezoelectric resonator 1d are connected to the pattern electrodes 6c and 6d, each being connected via the conductive bonding agent 7.
In the ladder filter shown in FIGS. 27 and 28, however, a land portion for connecting a conductive wire in a pattern electrode is required on the insulating substrate, which prevents a reduction in size of the filter. Also, reliability is a problem because a conductive wire might be cut by an impact. Furthermore, handling of a conductive wire is complicated, the cost of facilities is high, and materials of components, such as electrodes and pattern electrodes of a piezoelectric resonator, for allowing connection of a conductive wire, are limited.
In the ladder filter shown in FIGS. 30 and 31, the piezoelectric resonator is bonded to the pattern electrode via a conductive bonding agent, but the conductive bonding agent spreads out beyond the bonded portion of the piezoelectric resonator. For this reason, when the spacing between these piezoelectric resonators is made as narrow as possible in order to reduce component size, the electrode of the adjacent piezoelectric resonator touches the portion where the conductive bonding agent is spread, causing a failure due to a short-circuit, making a reduction in size difficult. For this reason, the spacing between the two piezoelectric resonators 1b and 1c in the center of the substrate must be a very large distance which prevents any decrease in size.